Apparatus and air separation plant

ABSTRACT

The present invention relates to an apparatus and air separation plant in which a pumped liquid oxygen stream is heated within a heat exchanger through indirect heat exchange with compressed air to produce an oxygen product. The liquid oxygen stream is pressurized in a range above about 55 bar(a) and no greater than about 150 bar(a) and is a supercritical fluid after having been heated within the heat exchanger. The air is compressed to an air pressure that is a function of the oxygen pressure that will result in a minimum power being expended in the compression of the air. The heat exchanger can be a brazed fin heat exchanger fabricated from aluminum in which the fins located in heat exchange passages have an undulating configuration to increase the flow path length and induce flow separation and thereby increase the heat transfer coefficient within the heat exchanger.

RELATED APPLICATIONS

This application is a continuation application of prior continuationapplication Ser. No. 12/842,098, filed Jul. 23, 2010, which is acontinuation of application Ser. No. 12/648,775, filed Dec. 29, 2009,which is a continuation of, and claims priority from, application Ser.No. 12/363,279, filed Jan. 30, 2009. All of which are incorporated byreference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to an apparatus and air separation plantfor forming an oxygen product as a supercritical fluid by heating apumped liquid oxygen stream within a heat exchanger through indirectheat exchange with compressed air. More particularly, the presentinvention relates to such an apparatus and air separation plant in whichthe air pressure utilized in to heat the pumped liquid oxygen stream isselected on the basis of a function of the oxygen pressure that resultsin a minimum or very close to minimum expenditure of compression energy.Even more particularly, the present invention relates to such aapparatus and air separation plant in which the heat exchanger is aplate-fin heat exchanger.

BACKGROUND OF THE INVENTION

There exists an increasing need for systems that are capable ofsupplying oxygen at very high pressures in which the oxygen exists as asupercritical fluid, namely, a fluid that is neither a vapor, solid orliquid, but is rather a dense fluid having a temperature and pressureabove the supercritical point. For oxygen, this temperature and pressurewould be above 154.78 K and 50.83 bar (a).

One reason for this increasing need is in the growth of gasificationapplications. Gasification is an environmentally friendly technologywhich can utilize coal or other relatively low value feedstocks andconvert them into high-value products, or alternatively produce a cleansource of electrical power by gasifying the feedstock within gasifiersinto hydrogen and carbon monoxide containing streams. These gasifierstypically require oxygen at high pressures in which the oxygen issupplied as a supercritical fluid. Although there are many differenttypes of gasifiers generally speaking, a low-grade carbon containingmaterial in the presence of oxygen is converted to a hydrogen and carbonmonoxide containing stream that can be further processed to be used as afuel in the generation of electricity and/or as a source of hydrogen, orfurther processed to manufacture valuable products such as chemicals,fertilizers or liquid fuels. Additionally, steam is generated in suchprocessing that can be further used to drive generators.

While such oxygen can be supplied by vaporizing liquid oxygen and thencompressing the oxygen to pressure, the liquid oxygen can be pumped to ahigh pressure and then heated to a critical temperature at which theresulting oxygen product will exist as a supercritical fluid. Typically,the pumping operation is incorporated into a cryogenic air separationplant, although, it is possible that the pumping operation could beconducted independently of such a plant. In a cryogenic air separationplant that is used in producing the oxygen at pressure, air iscompressed, purified and then cooled to a temperature suitable for itsrectification in a distillation column system.

Although different distillation column systems exist for therectification of air, a common system involves two columns, a highpressure column and a low pressure column that are thermally linked bymeans of a condenser reboiler. The air, after having been cooled to ator near its dew point, is then introduced into the high pressure columnin which nitrogen is separated from the air to produce a nitrogen-richcolumn overhead and a crude liquid oxygen column bottoms. The crudeliquid oxygen column bottoms is further refined in the low pressurecolumn into an oxygen-rich liquid column bottoms and a nitrogen-richcolumn overhead. All or part of the nitrogen-rich column overheadproduced in the high pressure column is condensed against boiling theoxygen-rich liquid column bottoms of the low pressure column to providea reflux for both high pressure column and the low pressure column.

The liquid oxygen that is drawn from residual oxygen-rich liquid in thelow pressure column is pumped to pressure and then heated in amulti-stream main heat exchanger that is used in cooling the air againstone or more product streams, or in a separate heat exchanger dedicatedto the heating of the oxygen. In either case, part of the air to berectified is further compressed in a booster compressor and then used toheat the oxygen and then produce the high pressure oxygen product thatcan be used in a gasifier or other process requiring high pressureoxygen.

As can be appreciated from this discussion, the raw material used inproducing the oxygen is the electrical power drawn, or steam consumed orfuel burned to produce the energy for compressing the air in the firstinstance and further compressing the air to vaporize the pumped oxygen.In this regard, since the cryogenic rectification is conducted atcryogenic temperatures and there exists thermal loss due to heatleakage, liquid products that are removed from the plant for storage,backup or merchant liquid sale and warm end losses, refrigeration mustbe imparted. This is commonly accomplished by further compressing partof the air to be separated and then expanding the air in a turboexpanderwith removal of the work of expansion. The resulting exhaust is thenintroduced into the distillation column system. There are other knownprocesses for generating refrigeration in an air separation plant. Theproduction of refrigeration represents a further energy requirement ofthe plant.

In order to produce oxygen at supercritical pressures, that is abovepressures at which the oxygen will exist as a supercritical fluid whenalso, at a temperature that will set the physical state of the oxygen asa supercritical fluid, the energy expended in compressing the air mustbe at a minimum or near a minimum to make the production of the oxygeneconomically attractive. In 89 AIChE Symposium Series 294, “ModernLiquid Pump Oxygen Plants: Equipment and Performance”, No. 294 by W. F.Castle, BOC Process Plants, p 14, it is mentioned that as a rule ofthumb, the pressure of the air has to be about 2.3 times that of theoxygen pressure that is required. A simulation was conducted over arange of oxygen pressures in which the oxygen was vaporized bycompressed air in a heat exchanger operated at a 5° C. warm endtemperature difference and at an approach or pinch of the heating andcooling curves of about 1.5° C. The results were presented in graphicalform. In the curve shown in the graph, at an oxygen pressure at above 40bar, the curve flattened out from a relationship in which the requiredair pressure was roughly twice the oxygen pressure. It was mentioned,however, in the paper that such curve did not represent optimumconditions for the best power consumption of the plant and such optimumconditions were not presented in the paper. It was also mentioned, thatthe heat exchanger for air pressures below 100 bar could be aconventional brazed aluminum plate-fin heat exchanger. However, athigher pressures, more expensive coiled heat exchangers would have to beused.

U.S. Pat. No. 6,430,962-B2 also considers the production of oxygen as asupercritical fluid. In this patent, the oxygen produced in a lowpressure column of an air separation plant is pumped to a supercriticalpressure and then vaporized in a brazed aluminum plate-fin heatexchanger. It is mentioned that the more narrow the temperaturedifference between the oxygen and the air at the warm end of the heatexchanger, the lower the thermal stress within the heat exchanger. Twocases were compared, one at 0.61 Mpa less than the critical pressure ofoxygen, 5.043 MPa and another far above the critical pressure, apressure of 8.14 MPa. From the comparison, it was determined that at thesubcritical pressure, the warm end temperature difference was large, 40°C. and at the high pressure, the warm end temperature difference was 12°C. This lower temperature difference would reduce the thermal stress inthe heat exchanger and allow the use of a brazed aluminum plate-fin heatexchanger in such applications. However, nothing is said in this patentregarding the most efficient operation of the plant with respect to theelectrical power used in compressing the air. Further, there are nodetails given regarding the design of the heat exchanger itself.

In U.S. Pat. No. 7,219,719 B2, a brazed aluminum plate-fin heatexchanger design is disclosed that is designed to be used at oxygenpressures above 100 bar. In this patent, straight extruded fins are usedin the high pressure channels, having a sufficient thickness towithstand such high pressures. It is mentioned that the ratio of themean fin thickness to the geometric pitch, or spacing between adjacentfins is preferably greater than 0.2 and less than 0.8. However, as willbe discussed, such a design would lead to an inefficient heat exchangerwith respect to the size required to accomplish the necessary heatexchange between air and pumped oxygen streams. U.S. Pat. No. 6,951,245discloses another brazed aluminum plate-fin heat exchanger that employsstraight fins.

As will be discussed, among other advantages, the present inventionprovides a method of producing an oxygen product as a supercriticalfluid that involves heating a pumped liquid oxygen stream with the useof supercritical pressure air in which a relationship has beendetermined that will allow the power consumed by the air compressor tobe minimized and that can be used in connection with a heat exchangerdesign that will incorporate a more efficient fin design than disclosedin the prior art.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an apparatus for producingan oxygen product from a liquid oxygen stream having a purity of no lessthan about 90 percent by volume. In accordance with this aspect of thepresent invention, a pump is provided to pump the liquid oxygen stream,thereby to produce a pumped liquid oxygen stream. A compressor isprovided to compress air and thereby to produce a compressed air streamand a heat exchanger is connected to the pump and the compressor suchthat the pumped liquid oxygen stream is heated within a heat exchangerthrough indirect heat exchange with at least the compressed air streamto produce the oxygen product. In this regard, the term “at least” asused herein and in the claims in this context, is meant to cover abanked heat exchange process in which the only heating stream is the airstream or alternatively a heat exchanger in which there might be otherstreams that would serve a heating function, albeit at a much lesserextent, such the main air stream and boosted streams to be passed into aturboexpander to generate refrigeration in an air separation plant. Thepump is configured to pump the liquid oxygen stream so that the pumpedliquid oxygen stream is pressurized to an oxygen pressure in a rangeabove about 55 bar(a) and no greater than about 150 bar(a) upon enteringthe heat exchanger. The compressor is configured to compress the air sothat the air is compressed to an air pressure upon entering the heatexchanger equal to about a value given by an equation in which the airpressure=0.00003×(oxygen pressure)³−(0.01141×(oxygenpressure))+2.263×(oxygen pressure)+2.5175. The heat exchanger isconfigured such that the pumped liquid oxygen stream is heated withinthe heat exchanger to a temperature at which the oxygen product will bea supercritical fluid. As used herein and in the claims, the“temperature” is any temperature at or above the supercriticaltemperature of oxygen which will be at and above 154.78 K.

As will be discussed, when the air stream is compressed to the airpressure in accordance with the equation above, the energy ofcompression will always be at or very close to a minimum at a given heatexchanger duty. It is to be also noted that although such a heatexchanger could be free-standing to deliver oxygen at pressure from aliquid source, the heat exchanger could be incorporated within an airseparation plant. In such case, the air pressure, for reasons that willalso be discussed, although not necessarily at the minimum could beequal to a value within a range of no less than 10 percent below and 20percent above the quantity determined from the equation set forth above.In this regard, in another aspect, the present invention provides an airseparation plant for producing an oxygen product. In accordance withthis aspect of the present invention, a compressor is provided tocompress the air and a pre-purification is connected to the compressorto purify the air and thereby to produce a compressed and purified airstream. A booster compressor is connected to the pre-purification unit.At least one heat exchanger is connected to the pre-purification unitand the booster compressor and is configured such that part of thecompressed and purified air stream is cooled within the at least oneheat exchanger and a further part of the compressed and purified airstream is compressed in the booster compressor to form a compressed airstream that is cooled within the at least one heat exchanger. An airseparation unit is connected to the at least one heat exchanger so as toreceive the part of the compressed and purified air stream and thecompressed air stream after having been cooled and is configured torectify the air and thereby produce a liquid oxygen stream having anoxygen purity of no less than about 90 percent by volume.

A pump connected to the air separation unit to pump the liquid oxygenstream, thereby to produce a pumped liquid oxygen stream. The at leastone heat exchanger is positioned between the pump and the boostercompressor and is also configured such that the pumped liquid oxygenstream is heated within the at least one heat exchanger through indirectheat exchange with at least the compressed air stream to produce theoxygen product at a supercritical temperature, at which the oxygenproduct will be a supercritical fluid and the compressed air stream willbe a liquid. The pump is configured to pump the liquid oxygen stream sothat the pumped liquid oxygen stream is pressurized to an oxygenpressure in a range above about 55 bar(a) and no greater than about 150bar(a) upon entering the heat exchanger. The booster compressor isconfigured to compress the compressed air stream so that the compressedair stream has an air pressure upon entering the at least one heatexchanger equal to a value within a range of no less than ten percentbelow and no greater than 20 percent above a quantity equal to0.00003×(oxygen pressure)³−(0.01141×(oxygen pressure))+(2.263×(oxygenpressure)+2.5175.

The at least one heat exchanger can be a first heat exchanger and asecond heat exchanger. The first heat exchanger is positioned betweenthe pre-purification unit and the air separation unit and is configuredto cool the part of the compressed and purified air stream. The secondheat exchanger is positioned between the booster compressor and the pumpand is configured to cool the compressed air stream and to warm thepumped liquid oxygen stream.

It is further noted that with respect to the prior art discussed aboveand as graphically presented in Castle, a meaningful benefit is derivedat an oxygen pressure of above about 55 bar (a). The reason for this isat lower oxygen pressures, the air pressure required to vaporize theoxygen, as given by the equation set forth above, is within 10 bar orless of an air pressure contemplated in such prior art. However, theinventors herein have calculated that such a 10 bar differencecorresponds to a unit power improvement of operating within suchequation of at most about 0.08 kW per 1000 cfh of oxygen produced whichwould translate into a decrease of about 1 percent of the power expendedin a booster air compressor used in an air separation plant. This inturn would represent a decrease in the overall power expenditure in theair separation plant of less than one-half a percent. As one skilled inthe art would recognize, from at least a financial standpoint, given thecost of electrical power, this is not a meaningful operationalimprovement. However, at oxygen pressures of above 55 bar (a) whenoperating at air pressures derived from the above equation much moremeaningful unit power improvements can be obtained and the improvementsare greater as the oxygen pressure increases. For example at 80 bar(a)oxygen, the improvement in power consumption of the booster aircompressor would be about 4 percent and therefore an improvement in theoverall power consumption of the air separation plant of about 2percent. At the high end of the range, 150 bar(a) oxygen, the risksassociated with operating a heat exchanger able to withstand therequired air pressure outweigh any power benefit leading to the use ofpressures lower than that given by the above equation albeit at a higherpower consumption.

In any aspect of the present invention, the indirect heat exchangebetween the pumped liquid oxygen stream and the air can be conducted ina plate-fin heat exchanger. In this regard, in yet a further aspect ofthe present invention, a plate-fin heat exchanger is provided thatcomprises parting sheets separated by and connected to fins to form atleast air passages for a compressed air stream and oxygen passages for apumped liquid oxygen stream. In this regard, such air and oxygenpassages are “at least” formed in that, as indicated above, the presentinvention is equally applicable to a heat exchanger dedicated to theheating of the pumped liquid oxygen. For example, in the air separationplant discussed above where a second heat exchanger is employed in theso called “banked” design, it can be dedicated to the heating the pumpedliquid oxygen stream. The fins in at least the air passages have anundulating configuration. As can be appreciated, such a heat exchangeris configured to withstand an oxygen pressure of the pumped liquidoxygen stream in a range above about 55 bar(a) and no greater than about150 bar(a) upon entering the heat exchanger and an air pressure of thecompressed air stream, upon entering the heat exchanger, equal to avalue within a range of no less than ten percent below and no greaterthan 20 percent above a quantity equal to 0.00003×(oxygenpressure)³−(0.01141×(oxygen pressure))+(2.263×(oxygen pressure)+2.5175.

In any aspect of the present invention, the fins in at least the airpassages can be provided with a wavy or undulating configuration suchthat the flow path of the compressed air through the fins is increasedover a straight through plain fin arrangement with the same finthickness and pitch.

Preferably, the undulating configuration can have regular spaced pointsof maximum amplitude along a length dimension of each of the finsforming peaks and troughs of arcuate configuration. The peaks and thetroughs are connected by straight segments of each of the fins. Thewavelengths of the fins are preferably equal to about in a wavelengthrange no less than about 0.125 inches and no greater than about 1.5inches.

When the oxygen pressure is at least about 80 bar(a), the air passagesand the oxygen passages can have an identical configuration. The finshave a maximum amplitude greater than a pitch dimension as measuredbetween adjacent fins. The fins can have a ratio of transverse thicknessto the pitch dimension which is greater than about 0.4 multiplied by afactor that is equal to the air pressure divided by an allowable tensilestress equal to about the yield stress for a material forming the heatexchanger multiplied by a safety factor of not greater than about 0.5and no less than about 0.15. The heat exchanger in any aspect of thepresent invention can be of brazed aluminum construction.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims distinctly pointing outthe subject matter that Applicants regard as their invention, it isbelieved that the invention will be better understood when taken inconnection with the accompanying drawings in which:

FIG. 1 is a graph of oxygen pressure versus air pressure in accordancewith a preferred embodiment of the present invention that also comparessuch relationship with that shown in the prior art;

FIG. 2 is a schematic diagram of an air separation plant in which anoxygen product is pumped and vaporized;

FIG. 3 is a heat exchanger in accordance with the present invention;

FIG. 4 is a schematic, sectional view of the heat exchanger shown inFIG. 3 taken along line 3-3 of FIG. 3;

FIG. 5 is a fragmentary, plan view of an arrangement of fins in the heatexchanger shown in FIG. 3 with portions of such heat exchanger brokenaway; and

FIG. 6 is a fragmentary, elevational transverse view of FIG. 5.

DETAILED DESCRIPTION

With reference to FIG. 1, the illustrated curve shown in a solid linerepresents the air pressure required to heat pumped liquid oxygen inaccordance with the present invention at a particular oxygen pressure toproduce an oxygen product as a result of the pumping and the heating asa supercritical fluid. When such air pressure is used, the minimumcompression power will be obtained for a particular oxygen pressure.

As would be known in the art, the power expended in compressing the airhas two components, namely, the pressure to which the air is to becompressed and the flow rate of the air. The air pressure and flow ratein turn must be sufficient to heat the oxygen at a specified flow rateand pressure from a pressurized liquid to a supercritical fluid afterhaving passed through a heat exchanger. Obviously, the lower the flowrate of the air, the higher the required pressure and vice-versa that isrequired for a particular flow and pressure of the oxygen. Althoughthere are many combinations of pressure and flow rate of the compressedair stream that will achieve the desired objective, it was found by theinventors herein that a specific air pressure exists for a particularpressure of oxygen to be heated that will always result in the lowestpower expenditure by the air compressor when the flow rate of the air isvaried to meet the thermal requirements in heating the pressurizedliquid oxygen stream to an ambient temperature. In this regard, whilethe actual ambient temperature used in the calculations was 24.4° C.(297.55 K), the actual value of the ambient temperature would not changethe results presented in the graph of FIG. 1 nor would the actualtemperature of the incoming liquid oxygen, which was assumed to be atabout −177° C. (96.15 K). Furthermore in any compression system,including that assumed for the calculation, the heat of compressionwould be removed from the air by an after-cooler, typically using waterto cool the air, to the ambient temperature level prior to entering theheat exchanger. However, as with the flow rates, none of thesetemperatures would have any effect on the results presented in FIG. 1.Instead, such variables as flow rate and entering temperature would havean effect on the heat exchanger design used to effectuate the indirectheat exchange between the air and the oxygen.

The required flow rate of the air will depend upon the flow rate of theoxygen and the design of the particular heat exchanger used. Put anotherway, the flow rate of the air is dependent on a product of the overallheat transfer coefficient and the heat transfer area (“UA”) and the logmean temperature difference. In any heat exchanger, the variation isdependent upon a minimum approach of the heating and cooling curves,known as the “pinch”, which optimally should be no less than 1.0 K. Whenthe pinch gets too tight, it becomes difficult to achieve the particularheat exchange desired in that small flow variations will have a largeeffect on the process. For an air separation plant, in order to make theplant self-sustaining without the need to add further refrigeration,another practical constraint is the warm end temperature difference atthe warm end of the heat exchanger which should be practically no morethan about 5 K. It is to be noted that the air compression used inboosting the air to a sufficient pressure to vaporize the pumped liquidoxygen represents about 30 percent of the power consumed by an airseparation plant and hence, such power is very significant. All of thisbeing said, the warm end temperature difference and the pinch will haveno effect on the air pressure derived from FIG. 1.

Another result of FIG. 1 is the smoothness of curve 1 and that it alsoencompassed subcritical pressures of oxygen to be compressed that havebeen verified for actual optimized operating pressures of existing airseparation plants. At these low pressures, the curve is fairlycoincident with the results in the prior art and in particular, Castlediscussed above. However, unexpectedly, a great divergence of theillustrated curve of FIG. 1 and that shown in Castle is seen atsupercritical pressures of oxygen. As mentioned above, the prior artcurve illustrated in this reference shows a change in the shape of thecurve that would be expected given that within the heat exchanger, theoxygen is transitioning from a pressurized liquid state at the cold endto a dense, supercritical fluid at the warm end. The inventors hereinhave found that such change in state has no effect on the optimum powercurve shown in FIG. 1. While not wishing to be limited to a particulartheory of operation, it is believed by the inventors herein that themost efficient power to be expended by a compressor compressing the airversus the heat energy required to warm the pumped liquid oxygen is justthat, a function of energy and nothing more, for example, adiscontinuity founded upon considerations relating to a change inphysical state.

In order to suitably quantify the present invention in a manner in whichthe current invention could be applied, the points making up theillustrated curve were generalized by polynomial curve fittingtechniques in which it was found that the most efficient air pressure,from the standpoint of electrical power input to a compressor, requiredto vaporize the pressurized liquid oxygen is given by an equation inwhich the air pressure=0.00003×(oxygen pressure)³−(0.01141×(oxygenpressure))+(2.263×(oxygen pressure)+2.5175. When this curve is used, itwas found that statistically, the variation of the curve from the actualpoints used in making the curve is roughly 0.995 or nearlyinfinitesimal. The actual points used in making up the curve are shownas small squares. Each square above 55 bar(a) represents a calculatedminimum power for the air at a particular oxygen pressure that wasdetermined by conducting a series of simulations around each point usingthe UNSIM DESIGN computer program that is offered by HoneywellInternational Inc. of Morristown, N.J., United States of America. Thepoints below 55 bar(a) were actual optimized points used in airseparation plants.

With reference to FIG. 2, a schematic diagram of an air separation plant1 that is used to make an oxygen product at supercritical pressures isillustrated. Although air separation plant 1 is an air expanded doublecolumn plant that is used to make oxygen and nitrogen products, thepresent invention would have application to any air separation plant inwhich a liquid oxygen product were produced and then pumped to asupercritical pressure. In this regard, while oxygen can be produced byair separation plants at a purity ranging from very low purity, about 90percent by volume to a high purity, above 99 percent by volume oxygen,the results presented in FIG. 1 would not be affected by a measurableamount with respect to oxygen purities of about 90 percent and above.Moreover, as indicated above, the present invention is equallyapplicable to any compression of air in heating pumped liquid oxygen.For example, instead of air separation plant 1, a stream of liquidoxygen might be obtained from a tank containing the liquid oxygen, suchstream would then be pumped and then vaporized in a vaporizer in whichthe compressed air were the heat transfer medium.

In air separation plant 1, an air stream 10 is compressed by acompressor 12 to produce a compressed air stream 14. Compressed airstream 14 is then passed through an after-cooler 16 to remove the heatof compression and is introduced into a prepurification unit 18.Prepurification unit 18 removes higher boiling contaminants in the airsuch as carbon dioxide, water vapor and potentially flammablehydrocarbons. The resulting compressed and purified air stream 20 isthen divided into first, second and third subsidiary streams 22, 24 and26.

First subsidiary stream 22 is fully cooled in a heat exchanger 28 to atemperature suitable for its rectification and then passed into an airseparation unit 30 that can consist of a high pressure distillationcolumn thermally linked to a low pressure distillation column toseparate the air into an oxygen-rich liquid stream 32 withdrawn from thebase of the low pressure column and a nitrogen-rich vapor stream 34withdrawn from the top of the high pressure column. Nitrogen-rich vaporstream 34 can be fully warmed to ambient temperature within heatexchanger 28 and then compressed in a product compressor 36 to produce anitrogen product stream 38. An impure nitrogen stream 40 can bewithdrawn from the low pressure column, below the nitrogen-rich vaporstream, and then divided into first and second portions 42 and 44. Firstportion 42 is fully warmed within heat exchanger 28 and a part 44thereof is used in regenerating adsorbent beds within theprepurification unit 18 and part 46 is discharged as a waste stream.

The second portion 24 of compressed air stream 20 is compressed in abooster compressor 48 and, after removal of the heat of compression inan after-cooler 50, is partially cooled to a temperature between thewarm and cold ends of heat exchanger 28 and is introduced into aturboexpander 52 to produce an exhaust stream 54. Exhaust stream 54could be introduced into the low pressure column to impart refrigerationinto the air separation plant 1. As illustrated, turboexpander 52 iscoupled to compressor 48 to drive the same with the work of expansion.It is also possible that the exhaust stream 54 be introduced into thehigh pressure column to impart the refrigeration. Nitrogen or wasteexpansion is also possible.

Third portion 26 of the compressed air stream 20 is introduced into abooster compressor 56 and, after removal of the heat of compression inan after-cooler 58, forms a compressed air stream 59 that is fullycooled within a heat exchanger 60 into a liquid stream 62. Thecompressed air stream 59 is the compressed stream that is used inheating a pumped liquid oxygen stream 64 that is formed by pumpingoxygen-rich liquid stream 32 in a pump 66 and thereby producing anoxygen product stream 68. Pumped liquid oxygen stream 64 has a pressurethat is above about 55 bar (a) which is above the critical pressure. Assuch, upon fully warming the pumped liquid oxygen stream 64, theresulting oxygen product stream at ambient temperatures is asupercritical fluid. It is to be noted that in lieu of the heatexchangers 28 and 60, a common heat exchanger could be used. Such a heatexchanger would have no effect on the optimum air pressure calculated onthe basis of the data presented in FIG. 1.

Although not illustrated, the liquid stream 64 is expanded, either in aliquid expander to generate additional refrigeration or in an expansionvalve so that the liquid can be introduced into the columns. Theresulting liquid after expansion could be divided into two portions forintroduction into intermediate locations of the high and low pressurecolumns. Second part 44 of the waste stream 40 is fully warmed withinheat exchanger 60 and discharged as another waste stream 70. As wouldalso be known to those skilled in the art, second part 44 of waste steam40 is used to thermally balance the heat exchangers 28 and 60 so thatthe difference between warm end temperatures of the streams exiting thelower pressure heat exchanger 28 and the higher pressure heat exchanger68 to inhibit warm end losses of refrigeration by such heat exchangersand also to decrease the temperature difference of the liquid stream 62and the first portion 22 of the compressed and purified air stream 20 atthe cold end of the high pressure heat exchanger 60 and the low pressureheat exchanger 28. In this way, the temperature difference between theliquid stream 60 and the pumped liquid oxygen stream 64 at the cold endof the higher pressure heat exchanger 60 can be optimized. It isadvantageous to decrease the temperature difference at the cold end ofthe higher pressure heat exchanger 60 in that the boosted pressure airliquefies within such heat exchanger and then thereafter, must beexpanded for its introduction into at least the lower pressure columnbut also, potentially, the higher pressure column. If the temperature ofthis stream is too warm, vapor will evolve from the boosted pressure airduring the expansion to have a deleterious effect on the requisitedistillation of the air to produce the desired products.

In carrying out the present invention, compressed air stream 59 uponentering heat exchanger 60 has a pressure determined in a mannerindicated in FIG. 1 for a particular pressure of pumped liquid oxygenstream 64. However, in certain circumstances it might be necessary tocompress second portion 26 of compressed and purified air stream 20either above or below the pressure determined in FIG. 1. For example, ifmore liquid were required, it would be necessary to produce compressedair stream 59 at a pressure above that determined in FIG. 1 to generatemore refrigeration within a liquid expander. By the same token, thereare situations in which it is desired to form compressed air stream 59at a pressure below the value determined in FIG. 1. In the illustratedembodiment, a specific heat exchanger design for heat exchanger 60 thatwould be capable of operating at a higher pressure than heat exchanger28 would be more expensive. Costs could be saved by operating heatexchanger 60 at a lower pressure. In either of these two situations,efficiency with respect to booster compressor 56 would be lost. At ahigher pressure, more energy would be expended than would be necessaryto warm the pumped liquid oxygen stream 64 and at a lower pressure, theflow rate of second portion 26 of compressed and purified air stream 20would have to be increased and as a result more energy would also beexpended in power booster compressor 56. However, there are practicallimits on this. One would not want to operate in a less than efficientmanner by more than about 1 percent of the power associated withcompression of the product oxygen flow. It was found that uniformly,operations that are conducted within a range of no more than ten percentbelow and 20 percent of the air pressure for compressed air stream 50 asdetermined from the solid line in FIG. 1 will be well within the 1percent efficiency differential. In FIG. 1, the dashed line locatedabove the solid line represents the pressure that is about 20 percentabove and the dashed line located below the solid line represents thepressure that is about 10 percent below the air pressure derived fromthe solid line.

There are additional reasons for not operating exactly on the curveillustrated in FIG. 1. For example, most heat exchangers such as heatexchanger 60 are oriented in a vertical position. As such, there is apressure loss as the oxygen ascends within such heat exchanger and again of liquid heat for the air that is liquefied at the bottom of theheat exchanger. Moreover, the efficiency is that of booster compressor56 and not necessarily that of the entire air separation plant. Asmentioned above, roughly 30 percent of the incoming air is sent tocompressor 56. The exact amount can be balanced with the amount of airsent to booster compressor 48 for purposes of generating refrigeration.For example, if heat exchanger 60 incorporates a more compact design,the warm end temperature difference will be greater requiring anadditional amount of refrigeration to be generated. At the same time,however, the more compact heat exchanger would be less expensive tobuild than a larger heat exchanger. Thus, the pressure selected forbooster compressor 56 might under those circumstances be higher thanthat predicted by the curve shown in FIG. 1 given the low flow tobooster compressor 56.

An air separation plant having the features of the air separation plantillustrated in FIG. 2 was simulated with the use of the UNISIM DESIGNcomputer program and at a series of oxygen pressures, an air pressurewas found for each oxygen pressure that produced a minimum unit power.Table 1, set forth below, shows such calculation for a pumped liquidoxygen stream 64 pumped to 100 bar(a) and a flow rate of 5326 kcfh. Asillustrated, the minimum unit power for booster compressor 56 occurs at138 bar(a) (2000 psia).

TABLE 1 Warm End Pressure Pressure Unit Power Temp. Diff UA Bar (a) psia(KW/Kcfh) Pinch K (WEDT)K Btu/hK 124 1800 18.58 1.229 3.302 5.71E+7 1311900 18.56 1.182 3.598 5.71E+7 138 2000 18.55 1.021 3.929 5.71E+7 1452100 18.57 1.008 4.277 5.71E+7 152 2200 18.6 0.983 4.63 5.71E+7Table 2 illustrates the effect on the pressure when the UA is varied byabout 20 percent from the base case shown in Table 1. Again the minimumunit power for booster compressor 56 is found to be 138 bar(a) (2000psia).

TABLE 2 Warm End Temp. Pressure Pressure Unit Power Diff UA Bar(a) psia(KW/Kcfh) Pinch K (WEDT)K Btu/hK 131 1900 18.42 1.229 0.793 6.86E+7 1382000 18.41 1.182 0.773 6.86E+7 145 2100 18.44 1.021 0.695 6.86E+7 1311900 18.83 2.212 4.09 4.57E+7 138 2000 18.81 2.225 4.391 4.57E+7 1452100 18.82 2.125 4.388 4.57E+7As is apparent, holding all other factors constant, varying the UA bymaking the heat exchanger larger or smaller has no effect on the optimumpressure. What is affected is the pinch, the warm end temperaturedifference and the unit power for the booster compressor 56. For exampleat the case of a UA 20 percent less than the base case, the pinchbecomes 2.225 and the warm end temperature difference rises to 4.391.The unit power has increased to 18.81. As expected with a larger heatexchanger, the pinch and warm end temperature difference has decreasedalong with the unit power. However, such decrease is at the expense offabricating a larger heat exchanger. These results can be generalized inthat larger or smaller UA's would exist at higher and lower flow ratesand yet, the optimum pressure found in FIG. 1 for a particular oxygenpressure would not vary.

In the practice of the present invention, the oxygen pressure sets theair pressure in accordance with FIG. 1. Once this pressure is fixed, aheat exchanger is designed that will accomplish an efficient warm endtemperature difference to lower overall power requirements forcompressing the air, while balancing the capital cost of the heatexchanger. Although the use of such a plate-fin heat exchanger ispreferred, other designs could be used such as prior art spiral heatexchangers in connection with FIG. 1. However, in a heat exchanger ofthe present invention, a brazed aluminum plate-fin heat exchanger isused that unlike prior art high pressure designs that incorporate astraight fin structure, an undulating fin structure is provided forincreasing the flow path length generating turbulence and mixing in theflow and thereby to effectuate an efficient heat exchanger design. Heattransfer is enhanced with the use of such fins by extension of the flowpath length (more heat transfer surface), breaking of the boundary layeras a result of periodic changes of the flow direction and impingement ofthe flow on to the neighboring fin surface. The intensity of sucheffects depends on the fin pitch “P”, wave length “L”, amplitude “A” andfin thickness “T”. When the amplitude “A” is less than the fin pitch“P”, the channel flow path length is not increased, merely roughened.While this will enhance heat transfer somewhat, there will not be theenhancement that exists when amplitude “A” is greater than the pitch“P”.

Such a design, as generally outlined above, is preferably incorporatedinto a practical embodiment of heat exchanger 60 and is shown in FIGS. 3and 4. Heat exchanger 60 is in the form of a brazed aluminum fin heatexchanger. Such a heat exchanger has at least a series of oxygenpassages 72 for the oxygen to be warmed in the formation of the oxygenproduct stream 68, air passages 74 for the compressed air stream to befully cooled into the two phase stream 62 and nitrogen balance passages76 for passage of the part 44 of the nitrogen waste stream 40 forthermal balancing purposes. Each of the passages is formed betweenparting sheets 78 and sealed at opposite sides by blocks 80 and 82 atthe ends by end blocks that are not illustrated. The top and bottom ofsuch a heat exchanger is sealed by top and bottom cap sheets 84 and 86.It is to be understood, however, that FIG. 4 is a schematic and in apractical installation there would be many more passages than thoseillustrated.

The compressed air stream 59 and the pumped liquid oxygen 64 stream areintroduced into the oxygen passages 72 and the air passages 74 by inletheaders 88 and 90 and the oxygen product stream 68 and the liquid stream62 are discharged from the oxygen passages 72 and the air passages 74 byoutlet headers 92 and 94. Similarly, the part 44 of the nitrogen wastestream 40 is introduced into the nitrogen passages 76 and discharged aswaste stream 70 through inlet and outlet headers 96 and 98,respectively. All of such construction is conventional and well known inthe art.

Within the passages are fins 100. The fins 100 serve to maintain thestructural integrity of heat exchanger 78 and to provide a greatersurface area for heat transfer to occur. Fluids pass within passages 101located between fins 100. In the prior art, such fins are extrudedstraight sections. However, in accordance with the present invention andas more specifically illustrated in FIGS. 5 and 6, the fins 100 have anundulating configuration in order to impart flow separations to the flowthrough the passages and therefore a greater heat transfer coefficient.As discussed above, the velocity of the flow passing through theundulating fins 100, at least the air flow, but possibly the oxygen flowif such fin design is also used for the oxygen passages, is selected toproduce such flow separations that can either be within a transitionbetween laminar and turbulent flow or at turbulent flow. In this regard,the velocity is selected to produce a Reynolds number of greater thanabout 400 as defined at a temperature midway between the warm and coldend temperatures of the heat exchanger. Such Reynolds number would atleast produce flow within the transition region. Since Reynolds numberis a ratio of the product of mean velocity, hydraulic diameter and fluiddensity to the dynamic viscosity of the fluid, the calculation of therequired velocity is a simple calculation from such relationship. Thisundulating configuration has regular spaced points of maximum amplitudealong a length dimension 104 of each of the fins 100 that form peaks 106and troughs 108 of arcuate configuration. The purpose of the arcuateconfiguration is to eliminate pressure drop losses that would otherwisebe produced by excessive turbulence had the peaks and troughs been sharppoints. Straight segments 110 connect the peaks 106 and the troughs 108.

In accordance with the above discussion, the fins 100 have a maximumamplitude “A” greater than a pitch dimension “P” as measured betweenadjacent fins 100. In order to maintain structural integrity, the finshaving a transverse thickness equal to the pitch dimension “P” which isgreater than about 0.4 multiplied by a factor that is equal to the airpressure of compressed air stream 59 divided by an allowable tensilestress equal to about the yield stress for a material forming the heatexchanger multiplied by a safety factor of not greater than about 0.5and preferably not less than 0.15. A safety factor of 0.25 is typicallyused. Practical wavelengths “L” of each of the fins 100 is in awavelength range no less than about 0.125 inches and no greater thanabout 1.5 inches. It is to be noted that in the illustration, all of thefins are of identical design. However, for oxygen pressures of thepumped oxygen stream 64 that are less than about 80 bar (a) the fins 100within the oxygen passages 72 could be made thinner in that such finswould not be subjected to the same degree of stress induced by thecompressed air stream 59 in the air passages 74. Although notillustrated, it is also possible to employ a perforated material to formthe fins 100. The perforations provide added turbulence but at theexpense of some loss of structural strength. When the fins used forcompressed air and oxygen are the same thickness, pitch amplitude etc.,it is advantageous to use a perforated version of the fin for the oxygenlayers.

As an example of a fin design to be used in heat exchanger 60 when usedin a service discussed with respect to FIG. 1, that is a heat exchangercapable of heating oxygen pumped to a pressure of about 100 bar (a) andusing an air stream having a pressure of about 138 bar (a), such heatexchanger could incorporate fins having a pitch “P” of about 0.038″ anda thickness “T” of 0.016″. The fin height “H” will be in a range ofbetween about 0.1″ to about 0.4″. Assuming that the aluminum ALLOY 3003were used to construct the heat exchanger, the maximum allowable tensilestress would be 24×10⁶ Pa, which represents the ultimate tensile stress(UTS) multiplied by a safety factor of 0.25. As is apparent, thethickness to pitch ratio of the fin would be equal to 0.42. If a finthickness of 0.020 were used, the corresponding pitch would need to be0.05″ (20 fins per inch). However, the preferred mode is to use asmaller pitch and corresponding thickness to maintain the ratiothickness to pitch of greater than about 0.42. This is because thesurface area will be higher for higher pitch. The same fin design couldbe used for both the air and the oxygen passages 74 and 72 respectivelyin case of heat exchanger 60. The heat exchanger, for example, heatexchanger 60 would then be designed with respect to the number oflayers, the arrangement of layers and the flow area within each of thelayers in a manner well known to anyone skilled in the art.

While the present invention has been described with reference to apreferred embodiment, as will occur to those skilled in the art numerousadditions, omissions and changes can be made to such embodiment withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

We claim:
 1. An apparatus for producing an oxygen product from a liquidoxygen stream having a purity of no less than about 90 percent by volumecomprising: a pump to pump the liquid oxygen stream and thereby producea pumped liquid oxygen stream; a compressor to compress air and therebyto produce a compressed air stream; a heat exchanger connected to thepump and the compressor such that the pumped liquid oxygen stream isheated within the heat exchanger through indirect heat exchange with atleast the compressed air stream to produce the oxygen product; the pumpset to pump the liquid oxygen stream so that the pumped liquid oxygenstream is pressurized to an oxygen pressure in a range above about 55bar(a) and no greater than about 150 bar(a) upon entering the heatexchanger; the compressor set to compress the air so that the air iscompressed to an air pressure upon entering the heat exchanger equal toabout a value given by an equation in which the airpressure=0.00003×(oxygen pressure)³−(0.01141×(oxygenpressure))+2.263×(oxygen pressure)+2.5175; and the heat exchangerconfigured such that the pumped liquid oxygen stream is heated withinthe heat exchanger to a temperature at which the oxygen product will bea supercritical fluid.
 2. An air separation plant for producing anoxygen product comprising: a compressor to compress the air; apre-purification unit connected to the compressor to purify the air andthereby to produce a compressed and purified air stream; a boostercompressor connected to the pre-purification unit; means for coolingpart of the compressed and purified air stream and for cooling acompressed air stream formed from a further part of the compressed andpurified air stream compressed in the booster compressor; an airseparation unit connected to the cooling means so as to receive the partof the compressed and purified air stream and the compressed air streamafter having been cooled and configured to rectify the air and therebyproduce a liquid oxygen stream having an oxygen purity of no less thanabout 90 percent by volume; a pump connected to the air separation unitto pump the liquid oxygen stream, thereby to produce a pumped liquidoxygen stream; the cooling means comprising a heat exchanger positionedbetween the pump and the booster compressor and also configured suchthat the pumped liquid oxygen stream is heated within the heat exchangerthrough indirect heat exchange with at least the compressed air streamto produce the oxygen product as a supercritical fluid and thecompressed air stream as a liquid; the pump set to pump the liquidoxygen stream so that the pumped liquid oxygen stream will bepressurized to an oxygen pressure in a range above about 55 bar(a) andno greater than about 150 bar(a) upon entering the heat exchanger; andthe booster compressor set to compress the compressed air stream so thatthe compressed air stream will have an air pressure upon entering theheat exchanger equal to a value within a range of no less than tenpercent below and no greater than 20 percent above a quantity equal to0.00003×(oxygen pressure)³−(0.01141×(oxygen pressure))+2.263×(oxygenpressure)+2.5175.
 3. The air separation plant of claim 2 wherein: theheat exchange means comprises a first heat exchanger and a second heatexchanger; the heat exchanger is the second heat exchanger; and thefirst heat exchanger is positioned between the pre-purification unit andthe air separation unit and configured to cool the part of thecompressed and purified air stream.
 4. The apparatus of claim 1 wherein:the heat exchanger is a plate-fin heat exchanger comprising partingsheets separated by and connected to fins to form at least air passagesfor the air and oxygen passages for the pumped liquid oxygen stream; andthe fins in at least the air passages having an undulatingconfiguration.
 5. The apparatus of claim 4, wherein the undulatingconfiguration has regular spaced points of maximum amplitude along alength dimension of each of said fins forming peaks and troughs ofarcuate configuration, the peaks and the troughs being connected bystraight segments of each of the fins.
 6. The apparatus of claim 4,wherein the pump is configured to pump the liquid oxygen stream to anoxygen pressure of at least about 80 bar(a) and the air passages and theoxygen passages have an identical configuration.
 7. The apparatus ofclaim 5, wherein the undulating configuration has a wavelength in awavelength range no less than about 0.125 inches and no greater thanabout 1.5 inches.
 8. The apparatus of claim 7, wherein: the fins have amaximum amplitude greater than a pitch dimension as measured betweenadjacent fins; and the fins having a ratio of transverse thickness equalto a pitch dimension which is greater than about 0.4.
 9. The apparatusof claim 8, wherein the heat exchanger is of brazed aluminumconstruction.
 10. The air separation plant of claim 3, wherein: thesecond heat exchanger is a plate-fin heat exchanger comprising partingsheets separated by and connected to fins to form at least air passagesfor the air and oxygen passages for the pumped liquid oxygen stream; andthe fins in at least the air passages having an undulatingconfiguration.
 11. The air separation plant of claim 10, wherein theundulating configuration has regular spaced points of maximum amplitudealong a length dimension of each of said fins forming peaks and troughsof arcuate configuration, the peaks and the troughs being connected bystraight segments of each of the fins.
 12. The air separation plant ofclaim 10, wherein the pump is set to pump the liquid oxygen stream to anoxygen pressure of at least about 80 bar(a) and the air passages and theoxygen passages have an identical configuration.
 13. The air separationplant of claim 11, wherein the wavelengths of the fins is in awavelength range no less than about 0.125 inches and no greater thanabout 1.5 inches.
 14. The air separation plant of claim 13, wherein: thefins have a maximum amplitude greater than a pitch dimension as measuredbetween adjacent fins; and the fins having a ratio transverse thicknessto the pitch dimension which is greater than about 0.4.
 15. The airseparation plant of claim 14, wherein the second heat exchanger is ofbrazed aluminum construction.